I propose to study the host innate immune response to infection with the Gram-positive, facultative, intracellular bacterial pathogen, Listeria monocytogenes. The molecular details of the L. monocytogenes infectious cycle are well known, and defective mutants exist for each stage of infection. I will use cDNA microarrays and real-time, quantitative PCR to characterize the transcriptional response of primary murine bone marrow macrophages (BMM), isolated from wild type (C57BL/6) mice, over the time-course of infection by wild type or mutant L. monocytogenes. This will provide valuable information about the mechanisms used by host cells to control progressive infection, not only by Listeria, but also by intracellular pathogens in general. Toll-like receptor 2 (TLR2) has recently been identified as a key initial component in the host's recognition of and inflammatory response to Gram-positive bacteria. In order to explore the role of TLR2 in the host innate immune response to L. monocytogenes infection, I will characterize the transcriptional response of BMM isolated from tlr2-/- mice over the time-course of infection by wild type or mutant L. monocytogenes. I ultimately wish to determine the exact involvement of specific host cell factors, identified as significantly up- or down-regulated in gene expression analyses, in nathwavs activated at each state of L. monocytogenes infection.